Mercy is a top-tier support in Overwatch, and there are plenty of reasons why. One reason is that she has excellent mobility, as displayed in this heroic save by a Mercy player.
As the ally Mercy makes her descent into the abyss after being knocked off the control point, she is desperately looking to find an ally to fly to. Lucky for her, Mercy was on the the way to assist her, and made it just in time to give her the boost.
The maneuver could have went south quickly if timed incorrectly. The Mercy players needed the Fly boosts, alternating with each other in order to make the save happen. Should the first Mercy have been a fraction of a second too late on her jump, or the ally Mercy have been unaware, both would have plunged to their deaths.
This play isn't going to be seen in the competitive scene because of single hero restrictions, but it is fun to see these kinds of game interactions that are possible between the same characters.